In modern motor vehicles, vehicle-occupant protection systems such as airbags, side bags or seatbelt pretensioners are used to reduce the severity of the injuries to vehicle occupants which occur in the case of accidents. The triggering of the vehicle-occupant protection systems is carried out here by means of a plurality of acceleration sensors which are arranged in the motor vehicle and which measure the acceleration which respectively occurs in a predefined direction in the case of an accident. As the individual acceleration sensors have different spatial orientations, both the direction and the magnitude of the acceleration can be calculated from the output signals of the acceleration sensors, the vehicle-occupant protection systems being activated if the magnitude of the acceleration exceeds a predefined limiting value (for example 10 g).
German laid-open application DE 196 45 952 A1 discloses a control circuit which evaluates the measurement signals of a plurality of acceleration sensors in order to detect an accident of the motor vehicle and activates the vehicle-occupant protection systems as a function of the measurement signals. Furthermore, this publication discloses that the measurement signals of the various acceleration sensors are to be subjected to a plausibility test in order to detect a malfunction of individual sensors or of the control circuit. If the various acceleration sensors are arranged, for example, in a star shape in one plane, the sum of the positive or negative accelerations which are measured by the acceleration sensors must be equal to zero. If the composite signal of the acceleration sensors deviates from this value, a malfunction is present in one or more of the sensors or else in the control unit itself. The plausibility check, described above, of the measurement signals generated by the acceleration sensors therefore advantageously makes it possible to detect a malfunction of the sensors or of the evaluation unit which analyses the measurement signals of the acceleration sensors in order to detect an accident.
However, a disadvantage of the known control circuit described above is that when the measurement signals which are supplied by the acceleration sensors are subject to a plausibility check during normal operation the check may be subject to errors.
On the one hand, this can lead to undesired triggering of a vehicle-occupant protection system if an acceleration sensor outputs a high acceleration value owing to a malfunction and the plausibility check does not detect the malfunction of the sensor.
On the other hand, an error in the plausibility check can lead to a situation in which the triggering of the vehicle-occupant protection system is disabled when there is an accident although the acceleration sensors measure a high acceleration value.